1. Technical Field
This invention relates to the field of mounting an electronic component on a substrate, particularly on circuit boards. More specifically the invention relates to a method and composition for mounting an electronic component on a substrate and the device formed by using such a method and composition.
2. Background Art
An increasing need exists in the electronics and/or computer industry to provide electronic packages of components and substrates, wherein multiple components are joined to a single substrate, such as a circuit board, particularly a printed circuit board (PCB). In fabricating such packages, the components and substrate are typically tested individually prior to assembly. However, once the package is fully assembled, defective devices may still be discovered in subsequent testing of the overall package due to problems at the system level, that is, problems only apparent when certain components and the substrate operate together in their respective systems. Also, problems may become apparent due to improper alignment or orientation of a component on the substrate. In either case, the most common remedy of the defect is rework, that is, removal and replacement of a component with a new component or repositioning of the component.
The rework process is time consuming and can introduce new defects into the package, other components, and/or the substrate. Often, solder paste is used to attach components to a substrate. The soldered connections that result from the soldering process are conductive and durable, as desired, however, they are also difficult to rework. A component attached with solder is generally removed by using a soldering iron or hot gas removal tools. The goal is to remove selectively and reattach only the desired components without effecting the delicate components that neighbor the desired components on the substrate. Nevertheless, whenever a soldering iron, hot gas tools, or other heated tools are used it is extremely difficult to avoid exposing neighboring components to the heat as well. The heat exposure may damage a component, the substrate, or a solder connection or otherwise introduce another defect into the package. Accordingly, a removal process that does not require heat is needed.
In some packages, adhesives are used to attach components to the substrate. The adhesives are often either conductive or nonconductive epoxies, wherein conductive epoxies are used in a similar fashion to solder paste. That is, the conductive epoxy is applied to the substrate, the electrical leads of a component are juxtaposed with the epoxy, and then heat is usually required to cure the epoxy and create a sufficiently strong mechanical bond and sufficiently reliable electrical connection. When solder paste is used, a similar process is followed except that the heat causes the solder paste to flow into a molten state and harden after removal of the heat. The ability to later reflow the solder allows reworking of soldered components, however, existing technology does not provide a process for reworking cured conductive epoxy. Accordingly, even if subsequent testing showed that an epoxied package was defective, current practices would not allow for the replacement or repositioning of selected components. For this reason, the inability to rework conductive epoxy is presently considered a disadvantage of such technology. Accordingly, an attachment process that also allows reworking is needed.
Attempts have been made to remedy the problems discussed above, however, they have met only limited success. Adhesive tape and conductive resin have been used for components with peripheral leads, wherein the leads electrically connect to contact pads on the substrate surrounding the perimeter of the component. The adhesive tape is placed between the component and the substrate to provide a mechanical connection and the conductive resin is placed on the contact pads to provide an electrical connection with the peripheral leads. The conductive resin does not bond the leads to the pads, it simply provides a conduction path, thus, the component may be replaced or repositioned by removing it from the adhesive tape. Accordingly, the removal process does not require heat and the attachment process allows reworking, but the electrical connection has proven unreliable. Because the conductive resin does not provide a mechanical bond of the leads to the contact pads, the electrical connection may be easily compromised. The adhesive tape might be sufficient to bond the component to the substrate, but it is not sufficient to keep the leads electrically connected to the contact pads. Accordingly, an attachment process that provides a durable electrical connection is also needed.
Notably, the tape and resin process described above is applicable to a limited number of component types. For example, it would be impractical to use the process with a component having for its leads an array of conductive bumps between the component and the substrate, providing both an electrical and a mechanical connection. The difficulty of applying adhesive tape amongst an array of contact pads on a substrate is typically sufficient to discourage use of the tape and resin process. Accordingly, an attachment process applicable to a variety of components is also needed.
Thus, it can be seen from the above discussion that it would be an improvement in the art to provide a method for forming durable connections between a variety of components and substrates, wherein the connections do not require heat for reworking. In other words, the ability to rework is needed while providing durable and reliable connections, wherein the rework has little impact on the substrate and neighboring components and connections.
According to the present invention, a method is provided for mounting a component on a substrate by: applying a conductive adhesive on a contact pad joined to a substrate; aligning a component with the substrate such that at least one lead of the component is juxtaposed with the conductive adhesive; performing a partial cure of the conductive adhesive, such that an electrical and mechanical connection suitable for testing is formed; testing performance of the component; and performing a full cure of the conductive adhesive, such that a permanent connection is formed. The method may include additional steps, for example, applying a tacky film to the substrate and juxtaposing the component with the tacky film. Also, for example, when the testing shows a defective or misaligned component, the component may be adjusted in its position or replaced or repositioned by cold separation of at least one component lead from the partially cured conductive adhesive. Further, additional conductive adhesive may be applied, when needed, before replacement or repositioning of a component.
A composition is also provided for mounting a component on a substrate including a conductive adhesive applied on a contact pad, the conductive adhesive being partially cured, wherein the composition allows cold separation of the component from the substrate and may selectively be fully cured to permanently mount the component. By way of example, the conductive adhesive may also be partially cured and/or fully cured by heat exposure.
An apparatus is also provided including a component, a substrate, and a means for temporarily mounting and electrically connecting the component on the substrate. In one example, the temporary mounting means may also be a means for selectively mounting the component permanently on the substrate. A partially cured conductive adhesive is one example of both a temporary mounting means and a selectively permanent mounting means. Tacky film may also be used in combination with the conductive adhesive.